The 25's strategy is based on the fact that 4 x 25
100.=
When you are multiplying you are making groups of another factor. When one of the factors is 25 it is easier to group the groups in fours rather than ones.
Example:
17 x 25 You need to make 17 groups of 25 in your head. Instead think how many groups of four can I make out of 17, or what times four would get me close to 17? 17 divided by 4 is 4 remainder 1, or 4 x 4 = 16 one less than 17. For every group of four you make out of 17 you count 100 because 4 x 25 = 100. Therefore four groups of four 25's is 400. What about the remainder? The remainder will always be either 1, 2, or 3 in this case it is 1, so you simply add on 25, so the total is 400 + 25 = 425.
Steps
Example 33 x 25
Step 1 Divide 33 by 4 or ask what times four is close to 33
33 / 4 = 8 R1 or 4 x 8 = 32 one away
Step 2 multiply the quotient by 100
8 x 100 = 800
Step 3 Add on the remainder. R 1 = 25, 2 = 50, 3 = 75
The 25's strategy is based on the fact that 4 x 25
100.=When you are multiplying you are making groups of another factor. When one of the factors is 25 it is easier to group the groups in fours rather than ones.
Example:
17 x 25 You need to make 17 groups of 25 in your head. Instead think how many groups of four can I make out of 17, or what times four would get me close to 17? 17 divided by 4 is 4 remainder 1, or 4 x 4 = 16 one less than 17. For every group of four you make out of 17 you count 100 because 4 x 25 = 100. Therefore four groups of four 25's is 400. What about the remainder? The remainder will always be either 1, 2, or 3 in this case it is 1, so you simply add on 25, so the total is 400 + 25 = 425.
Steps
Example 33 x 25
Step 1 Divide 33 by 4 or ask what times four is close to 33
33 / 4 = 8 R1 or 4 x 8 = 32 one away
Step 2 multiply the quotient by 100
8 x 100 = 800
Step 3 Add on the remainder. R 1 = 25, 2 = 50, 3 = 75
800 + 25 = 825